1. Field of the Invention
The present invention relates to an antenna arrangement. More particularly, the present invention relates to an antenna arrangement associated with an aircraft collision avoidance system on a host aircraft for determining a direction from which a transponder reply signal is received from threat aircraft.
2. Description of the Related Art
Almost all active aircraft are equipped with a transponder that, when interrogated, transmit reply signals having coded information relating to the aircraft associated with the transponder such as the altitude of the aircraft, for example, Transponder-based aircraft collision avoidance systems rely on transmitted reply signals from the existing population of airborne transponders for operation. By receiving coded transponder reply signals at a host aircraft that are transmitted from a threat aircraft, the altitude separation between the host aircraft and the threat aircraft can be determined and a safe altitude separation can be maintained between the two aircraft for avoiding a disastrous encounter. A transponder-based aircraft collision avoidance system is disclosed in U.S. Pat. No. 5,077,673 and U.S. Pat. No. 5,157,615, both issued to Brodegard et al., both of which are incorporated by reference herein.
The effectiveness of a collision avoidance system can be further enhanced by determining the approximate distance from a host aircraft to a threat aircraft based on the strength of a received transponder reply signal. Accurate ranging to a threat aircraft is accomplished by a host aircraft actively interrogating a threat aircraft, and then measuring the time between transmission of an interrogation signal and reception of a reply signal.
Although knowledge of the direction, or bearing, to a threat aircraft relative to the heading of a host aircraft is not essential to have collision avoidance, knowledge of the bearing to a threat aircraft greatly enhances a pilot's ability for visually acquiring the threat aircraft and for having a better spacial perspective of threat aircraft relative to the host aircraft.
Directional antennas are used in some transponder-based collision avoidance systems for determining bearing to a threat aircraft, but they are specialized and require complex circuitry for reliable operation. One common arrangement uses a quadrapole antenna array with output signals being combined so that the phase difference between two output ports of the combining circuitry are indicative of the bearing of a received transponder signal. This particular arrangement requires that the antenna cables be phase matched, and that the phase matching be maintained through two receiving channels of the system for accurate and reliable bearing determination. This requirement is both delicate and complex.
Another problem encountered with conventional aircraft collision avoidance systems using directional antenna is the location of the antenna on the airframe. Signals arriving at the antenna from directions that are "shadowed" by the airframe, that is, from directions which place the transmitter out of the line of sight from the antenna by virtue of the airframe blocking the antenna from the transmitter, are attenuated. Shadowing is presently overcome by placing a primary directional antenna on a top surface of the aircraft and a second antenna on a bottom surface of the aircraft. The bottom mounted antenna is usually omnidirectional. While using a single directional antenna reduces cost, it only does so at the expense of reduced directional coverage. To compensate for this drawback, the bottom antenna can be a duplicate of the top mounted directional antenna, but use of two directional antennas adds considerable cost and complexity to the aircraft collision avoidance system.